Cooking pizza last night. Temp would not rise so I used my wiggle rod as I have done many times before. Flames shot out of the bottom vent door and caught my jacket on fire. Temp rose instantly to over 800 put on the top vent and closed the bottom vent. Temp went back down to 200. Opened everything back up and about every 30 seconds or so flame shot out of the bottom door and each time the temp rose about 100 degrees. this went on for a while and finally everything stabalized at about 550 never had that happen before and I have had my large for about 12 years. What could cause this?